The objective of the proposed research is to advance and evaluate a real-time, intra-operative biomechanical guidance system (BGS) that may improve decision making during joint surgeries. The system will be used by the surgeon during periacetabular osteotomies (PAO). The BGS will interact with the surgeon during the operation and display the current biomechanical state of the joint. It will use the information from the imagery of the navigation system and display the joint orientation and contact pressure distribution in the hip joint as the joint alignment is modified intraoperatively. The proposed research will further enhance the BGS prototype technology by developing new cartilage segmentation and deformable registration techniques as well as novel tools and interfaces for surgeon-BGS interaction (Aim 1). The theoretical foundation of the BGS will be extended in order to model subluxation, joint incongruency in hip dysplasia, and nonlineaavers (Aim 4). Based on available conventional techniques, the minimum anatomical criteria for successful joint realignment during PAO surgery will be devised. The reliability of the system will be determined by testing whether the system continuously satisfies those criteria for joint realignment. The surgeon will use the biomechanical and virtual-radiographic feedback of the BGS to supplement the conventional PAO techniques in order to realign the anatomical angles of the dysplastic hip to that of the normal hip. In order to compare the clinical outcomes of the osteotomy with real-time BGS to osteotomy without BGS, annual clinical follow-ups will be performed and the clinical outcomes will be compared to the biomechanical and radiological outcomes (Aim 5). While the focus of this work will be on periacetabular osteotomy, applications of this research can be extended to other types of hip osteotomies, other joint osteotomies, and total joint replacement techniques. 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